The present invention relates to a multi-point measuring technology of a FBG sensor, and in particular, it relates to a multi-point measuring method of the FBG sensor for detecting deformation of a target to be measured and a multi-point measuring apparatus. In more details thereof, it relates to the multi-point measuring method of the FBG sensor and the apparatus for enabling detection without exchanging a fiber optical path, with providing plural pieces of optical fibers, being inserted plural numbers of FBG sensors in series, each of which changes a wavelength of a reflection light, and multiplexing them time-divisionally.
In recent years, as a means for measuring temperature or distortion of a target with using optical fibers is proposed a FBG sensor method, with using a Fiber Bragg Grating (hereinafter, being described “FBG”) therein. The FBG sensor forms diffraction gratings, repetitively, with a portion having a high refractive index and a portion having a low refractive index at a constant distance within a core of the optical fiber.
Explanation will be made on a principle of the general FBG sensor, by referring to FIGS. 13A to 13D attached herewith. FIG. 13A shows distribution of wavelength of an optical signal, which is inputted to the FBG sensor, and FIG. 13B shows a cross-section view of the FBG sensor, respectively. FIG. 13C shows distribution of the optical signal, which is reflected and outputted by the FBG sensor, among inputs inputted, and FIG. 13D shows distribution of wavelength of an optical signal, which passes through the FBG sensor, among the inputs inputted, respectively. An optical fiber 500 is constructed with a core 501, i.e., a fine material made of quarts glass, being disposed at a center, and a clad 502 covering over the periphery thereof. Since the quarts glass is brittle, it is covered by a protection film 503 on the periphery thereof. On the core 501 is formed a periodic diffraction grating 505 at a pitch “Λ” within a range of length “L”. This diffraction grating 505 can be formed through irradiating an interference pattern of ultraviolet light (of a wavelength of around 250 nm, for example,) upon the core 501 of the optical fiber, thereby changing the refractive index of the core 501, periodically. A part, on which this diffraction grating 505 is formed, is called the FBG sensor 5.
The wavelength of a reflection light 507 of the FBG sensor 5 results to change depending on a physical quantity, which changes the refractive index of the core 501 or the period of the diffraction grating 505. For example, if thermal change is caused in the FBG sensor 5, since a fluctuation of the refractive index is generated due to the thermal change of the core 501, and the period of the diffraction grating 505 is varied, then the wavelength of the reflection light 507 is shifted. An amount of the shift due to this temperature comes to be about 10 pm/° C., when applying a wavelength band of 1.5 μm. Also, distortion generated in the FBG sensor 5 brings about generation of expansion and contraction in the core 501, then the diffraction grating 505 changes the period or frequency thereof. The shift amount due to this distortion comes to about 1.2 pm/με.
A method is described in the following Patent Document 1, for example, wherein the FBG sensor is applied as a multi-point distortion and temperature sensor. In this method, plural numbers of FBG sensors, each changing the wavelength of the reflection light, are inserted within an optical fiber, in series, wherein an optical signal is entered from an end of this optical fiber to measure an amount of the reflection light by an OTDR (Optical Time Domain Reflectometer), and also to measure back scattering light from an inside of the optical fiber by the OTDR, and thereby obtaining distribution of temperature along the longitudinal direction of the optical fiber. And then, upon basis of this distribution of temperature, correction is made on an amount of change of the reflection light depending on the temperature of each FBG sensor, and from this corrected mount of the reflection light is obtained an exact distortion of each FBG sensor, in the method thereof. Thus, the OTDR is that for measuring a condition and a place of loss within the optical fiber, by measuring a position and an intensity of the back scattering light while entering an optical pulse into the optical fiber. The distance revolving power of thereof is, in general, about several ten centimeters. In case where plural numbers of pieces of the optical fibers are applied, the measurement can be made by one (1) set of a measuring instrument with exchanging them by using an exchanging switch therein. Further, it is said that the number of pieces of the optical fibers can be increased depending on a capacity of the exchanging switch.
Also, a method for providing plural numbers of the FBG sensors in a rotating body is described in the following Patent Document 2, for example. In this method, the optical signals of the FBG sensors, which are provided in the rotating body, are transmitted to a receiving portion, which is provided outside, in a non-contact or contactless manner. In case where an optical axis, through which an optical transmission is conducted, lies at the center of rotation, the transmission can be made with using only one (1) piece of a transmission path, and then, where the plural numbers of the FBG sensors are provided within the rotating body, it is mentioned, the optical fibers are exchanged with using the exchanging optical switches, so as to deal with that.